The velocity of the flow of blood in an artery, vein or heart of a patient has been measured by using an electroacoustical transducer to transmit bursts of several cycles of ultrasonic pressure waves having a carrier frequency of 2.pi.f.sub.c into the body along a line that intersects the flow at an angle of other than 90.degree.. A portion of the acoustic energy so transmitted is reflected back along the line toward the transducer by particles in the blood because they have a different acoustic impedance than the fluid in which they are immersed. The frequency of the pressure waves thus reflected depends on the component of the velocity of the particles along the line. In accordance with the Doppler principle, the frequency of the pressure waves thus reflected from a particle increases with the velocity at which it approaches the transducer; decreases with the velocity at which it recedes from the transducer; and remains the same if its component of velocity along the line is zero. For reasons understood by those skilled in the art, the average velocity of blood particles within a sample volume at a desired range can be determined by processing samples of electrical waves corresponding to reflections from blood particles in that sample volume. The size of the sample volume is proportional to the length of the transmitted burst in the body and inversely proportional to the azimuthal resolution of the transducer.
In a European patent application No. 83104067.0 published on 02.11.83 with a publication No. 0 092 841, the average velocity of the particles in a sample volume is determined as follows. The electrical waves at the output of the transducer that are respectively due to reflections of each of a given number of bursts from particles in a sample volume of interest are sampled at successive instants of time that are spaced by one-quarter of a cycle of the carrier frequency, 2.pi.f.sub.c, so as to produce quadrature phased real and imaginary components of the reflections of each burst. The quadrature phased components of one burst and the conjugate of the quadrature phased components of the next burst are multiplied. This is done for all adjacent bursts so that if only seven are involved, there are six pairs of multiplications. The real components from all the multiplications are averaged and the imaginary components are averaged. The angle having a tangent determined by the ratio of the average of the imaginary components to the average of the real components can be shown to be the average of the phase changes of the reflected waves occurring between each pair of successive transmitted bursts, and this can be translated into the average frequency of the reflections from the particles. The average velocity can be derived from the average frequency.
Because this is in effect a vector addition of the quadrature components of the reflections of each transmitted burst, the result is dependent on their amplitudes, and because in practice the amplitude of one set of quadrature components can be much greater than the others due to factors not related to the velocity of the particles of interest, they can dominate the averages so that the velocity determined from them can have a considerable error.